Picking up the most recent journals to arrive in my office (selection bias, one coder, etc.), I find the following:
American Political Science Review (May 2006): two articles by one author (each), six articles by two authors, two articles by more than two authors.
American Journal of Political Science (July 2006): ten articles by one author, four articles by two authors, five articles by more than two authors.
American Economic Review (June 2006): three articles by one author, ten articles by two authors.
Harvard Law Review (June 2006) (excluding student pieces): one book review by one author and one article by two authors (that, if I can shamelessly say, cites my dissertation work on the GAO, more on my research in an upcoming post).
Yale Law Journal (June 2006) (excluding student pieces and the colloquium): three tributes by one author, one article by one author, one essay by two authors.
Columbia Law Review (June 2006) (excluding student pieces): one essay by one author, one article by one author.
I would have thought the law review numbers on co-authored work would be even lower. Does anyone know what those numbers are, looking more systematically? How can law schools encourage more co-authored papers, especially for empirical work? How is co-authored work treated in the tenure process, in law or in other fields? Some of my colleagues did very little co-authored work as untenured faculty members but now, with tenure, routinely publish with others. I recall some thread on this topic in the past several months but could not find it with some quick web searching, so I apologize for any repetitive posting. (Raymond Sauer has an interesting piece, Estimates of the Returns to Quality and Coauthorship in Economic Academia, in the Journal of Political Economy (1988) (JSTOR link provided), on the effects of co-authorship on economists' salaries).